1. Field of the Invention
The present invention relates to a backlight unit and a display employing a backlight unit, and more particularly, to a backlight unit that provides a dynamic image by properly controlling the brightness of light emitting devices that can be driven independently with respect to an image having a large difference of brightness, and a display employing the backlight unit.
2. Description of the Related Art
Liquid crystal displays (LCDs) are used in notebook computers, desktop computers, LCD-TVs, mobile communication terminals, and so on. Since an LCD is a light receiving element type display that cannot emit light by itself, the LCD needs a backlight unit in addition to a liquid crystal panel. The backlight unit is located in the rear of the liquid crystal panel and emits light onto the liquid crystal panel.
The backlight unit can be classified as a direct light type backlight unit and an edge light type backlight unit in accordance with the arrangement of a light source. The direct light type backlight unit irradiates light from a plurality of light sources provided under the liquid crystal panel toward the liquid crystal panel. The edge light type backlight unit emits light from a light source located at a sidewall of a light guide panel (LGP) to the liquid crystal panel. A cold cathode fluorescent lamp (CCFL) is generally used as the light source for the edge light type backlight unit.
Meanwhile, a light emitting diode (LED) is considered as a substitute for the CCFL. For example, LEDs emitting Lambertian light are used as a point light source for the direct light type backlight unit.
Referring to FIG. 1, a conventional backlight unit includes an LED 500, a diffusion plate 503 and a diffusion sheet 505 for projecting light emitted from the LED 500 onto a liquid crystal panel 510 to be uniform, and a reflection plate 502 for reflecting light that is emitted from the LED 500 to proceed toward the liquid crystal panel 510 located above LED 500. Further, a prism sheet 507 is provided to correct a light travelling route between the diffusion sheet 505 and the liquid crystal panel 510 and cause the emitted light to proceed toward the liquid crystal panel 510.
In an LCD, however, a slow response time of the liquid crystal results in a motion blur phenomenon in a fast moving picture. Since the amount of light from a conventional backlight unit is identical over the entire surface of the LCD, an image is monotonous as a whole. For example, for an image that requires a partial increase of the brightness, as in an explosion scene, or an image that requires a partial decrease of the brightness, as in a starlit night sky as a background, there is a limitation in representing the images vividly.
Specifically, eight CCFLs arranged in a line are required for a 26-inch display, and sixteen CCFLs for a 32-inch display. In order to control the brightness, the CCFLs as a line light source need to control each current applied thereto. However, all CCFLs are connected in series and it is impossible to finely control a region needing the increase or decrease of the brightness, even though the CCFLs are driven independently. Consequently, the CCFLs cannot provide a dynamic image.
Also, in the case of a backlight unit using LEDs as a light source, all LEDs are connected and driven in series. Accordingly, it is possible to decrease or increase the overall brightness of LEDs, but it is impossible to increase and decrease the brightness properly.
As described above, because the conventional backlight unit cannot provide an image needing a partial increase or decrease of brightness, it is difficult to provide a dynamic image.